The interstitial lung disorders represent 15 to 20 percent of all pulmonary disorders; in most cases these diseases cause significant disability and many are fatal. Studies of the natural history, etiology, pathogenesis, pathophysiology and therapy of these disorders have made major inroads into understanding these diseases. Most importantly is the development of the concept that the inflammatory and immune effector cells are critical determinants in the pathogenic process. Methodologies have been developed to evaluate the alveolitis of these patients and to examine its effect on the alveolar structures, particularly the extracellular matrix. Thereapeutic trials are underway to evaluate the efficacy of drug programs aimed at irradicating the alveolitis of these diseases.